An etching process is provided for aluminum metal which produces a uniform surface with low reflectance and hides surface imperfections while at the same time increasing the whiteness of the surface. The etching process provides successive or multiple stage short-time etching treatments in which the first treatment employs special etching solutions based on alkali metal hydroxide, chelating agent and special oxidizing agents. The second or successive etching treatment employs conventional alkali metal hydroxide etching solutions. The multiple stage etching process produces aluminum metal suitable for architectural purposes without the necessity of using mechanical buffing prior to etching.

1. In the process of etching aluminum metal in alkali metal hydroxide solutions to produce low reflecting etched surfaces having a uniform appearance, the improvement comprising subjecting unbuffed aluminum metal to successive short-time etching treatments in which the unbuffed aluminum metal is contacted with a first aqueous alkali metal hydroxide etching solution comprising the following composition expressed in grams per liter of water:

2. The process of claim 1 in which the second aqueous alkali metal hydroxide etching solution comprises the following composition expressed in grams per liter of water:

3. The process of claim 2 in which the second aqueous alkali metal hydroxide etching solution contains at least 0.0002 percent by weight of a

4. The process of claim 1 in which the first aqueous alkali metal hydroxide aluminum etching solution comprises the following composition expressed in grams per liter of water:

5. The process of claim 1 in which the first alkali metal hydroxide etching

6. The process for etching aluminum which comprises subjecting the aluminum work piece to at least two successive etching treatments as provided by

7. The process of claim 1 in which the oxidizing agent in the first etching solution is selected from the group consisting of sodium, potassium and ammonium nitrite, and the concentration of the nitrite compound is 5 to

8. The process of claim 7 in which the first etching solution also contains a nitrate compound in an amount equivalent to 5 percent by weight of the

9. The process of claim 8 in which the nitrate compound is selected from

10. The process of claim 1 in which the aluminum metal is desmutted between the first and second etching treatments.

Description:

BACKGROUND OF THE INVENTION

The appearance of many aluminum products is often marred by scratches, handling or machining marks, oxidation and corrosion pits. Many such products are subjected to an etching treatment which produces a more uniform pleasing appearance on the aluminum surface by removing or covering surface imperfections.

Aluminum finishers, using the conventional available etching processes, find that they may have to etch their aluminum products for as long as 15 minutes to achieve the desired improvement in appearance. Such prolonged treatments add considerably to the cost of aluminum metal finishing, in that large amounts of metal may be removed, and large amounts of etching chemicals may be consumed by the prolonged etching.

The conventional aluminum etching solutions generally employ aqueous alkali metal hydroxide solutions and a chelating agent. The conventional etching solution usually comprises a strong sodium hydroxide solution of about 15 to 250 grams per liter and a chelating agent to keep the dissolved aluminum from redepositing sodium aluminate on the metal surface. These etching solutions normally produce a silver to silver gray metallic appearance. In addition to the silver color such surfaces will have a high reflectivity and may be shiny. In addition the etched surface will be non-uniform in character and there will be no uniformly grained appearance. The resulting overall surface will not hide the die or extrusion marks and scratches are often visible.

The conventional etching solutions remove large amounts of aluminum metal from the surface without providing a good hiding or obscuring of the surface imperfections. The surface produced by a chelated caustic solution treatment gives an etched surface slightly more pleasing than the raw metal while hiding only a small amount of surface imperfections.

Highly reflecting surfaces may be obtained by using a combination of high nitrate and low chlorite concentrations so as to produce a mirror surface in U.S. Pat. No. 2,673,143. Similar brightening effects were obtained in U.S. Pat. No. 2,671,717 by using high concentrations of nitrate.

As the demand for a better hide of surface imperfections increased a "frosty" etched finish was provided by various additions of phosphates and/or fluorides. This was an improvement over the chelated caustic etching, but did not gain wide acceptance because of insufficient hide of surface irregularities.

The next development in etching aluminum for architectural purposes such as for window and door framing and curtain walls was the use of chromates in etching solutions such as described in U.S. Pat. No. 3,314,890. This treatment provided frosty white matte etched finishes with good hide of surface imperfections. The chromate etching system also eliminated the problem of galvanizing.

The above etching processes removed as much as 5 percent by weight of the metal being etched because of the long etching cycles which ran from 10 to 15 minutes in contact with the caustic soda. This produces excessive chemical consumption of the etching solutions as well as creating serious disposal problems of the spent solutions containing tremendous amounts of dissolved aluminum metal.

To meet the ever-increasing demand for architectural aluminum which is non-reflecting and at the same time free of surface imperfections, the aluminum processor first mechanically buffed the aluminum to remove scratches, pits, die lines, rolling marks or any surface irregularity. This mechanical buffing left the aluminum with an undesirable, highly reflecting surface. Since the mechanical buffing is performed by hand labor it greatly increased the cost of preparing the aluminum. In addition, an etching treatment was required to provide low reflecting frosty white matte surfaces. This combination of buffing and etching produces a highly uniform low reflecting frosty white surface which meets present day standards for architectural aluminum. However, the combined buffing and long etching treatment produces a high loss of aluminum metal.

SUMMARY OF INVENTION.

I have now discovered a process for etching unbuffed aluminum metal with alkali metal hydroxide solutions which produces architectural aluminum characterized by having a surface frosty and white in color and free of irregularities and other imperfections. The etched surface is uniform in appearance with low reflectance and, under preferred etching conditions, the surface is non-image forming. This highly desirable finish is obtained without the necessity of mechanical buffing.

My improved etching process for unbuffed aluminum metal comprises successive short-time etching treatments employing as the etchant aqueous alkali metal hydroxide solutions. My invention is also characterized in that the first etching treatment takes place in alkali metal hydroxide solutions containing one or more particular oxidizing agents. The second etching treatment takes place in conventional alkali metal hydroxide etching solutions.

Generally, in my etching process only a first etch with the alkali solution containing the oxidizer and a second etch in the conventional etching solution are all the etching treatments that are required to produce frosty white low reflecting aluminum surfaces. However, additional etching cycles may be employed if necessary. The total etching time of a two etch-cycle is generally less than the time employed in a single etching of aluminum metal which has been mechanically buffed before etching.

The first etching solution comprises one or more oxidizing agents in solution with the alkali metal hydroxide. The second or successive etching treatment usually employs the conventional aqueous alkali metal hydroxide etching solution and need not contain the oxidizing agent of the first etching solution for reasons of economy. However, from the standpoint of surface obtained, the second etching solution may also contain one or more of the selected oxidizing agents.

The oxidizing agent which is employed in the first etching solution is selected from the group consisting of persulfates, peroxides, perborates, periodates, chlorites, chlorates, perchlorates, iodates, hypochlorites, nitrites, peroxyphosphates, and bromates. The oxidizing agent is used at concentrations of about 2 to about 120 grams per liter. The alkali metal hydroxide concentration will be at about 15 to about 50 grams per liter.

Special chelating agents are required for use in the etching solution containing the oxidizing agent and consist of one or more compounds selected from the group consisting of sorbitol, gluconic acid and gluco-heptonic acid and their alkali metal salts.

The first etching process using the oxidizing agent is operated at a temperature of about 130° to about 200°F. The preferred operating range is about 150° to about 170°F. In general, the higher the operating temperature, the faster is the rate of etching. The second or successive etching using the conventional etching solution is operated within the range of about 120° to about 190°F.

Between the successive etching treatments the aluminum is rinsed with water and then desmutted if necessary as shown in U.S. Pat. No. 3,275,562 to remove surface impurities not attacked by the caustic soda. If desmutting is unnecessary, the aluminum work piece goes directly to the second etching treatment following a water rinse. The improved etching process may comprise successive or multiple stage etching treatments for aluminum such as contacting unbuffed aluminum with (1) etching solution containing oxidizing agent, (2) a conventional etching solution, (3) an etching solution containing oxidizing agent and finally, (4) a conventional etching solution.

After the second or final etching treatment, the work piece is rinsed with water, desmutted and rinsed again with water. Thereafter, it may go to an anodizing process or to paint preparation processes.

Both the etching solution containing the oxidizer of the first etching stage and the conventional alkali metal hydroxide etching solution of the second or successive stage may contain other additives such as aluminum surface modifying agents. These may be surfactants such as fluoroalkyl surfactants as shown in U.S. Pat. No. 3,557,000 which provide long-lasting bath life. As used hereafter in the specification and claims the term fluoroalkyl surfactant means a fluorine containing surfactant as disclosed and claimed in U.S. Pat. No. 3,557,000. Special surfactants to improve rinsing and to reduce fuming and foaming, fluorides and phosphates which produce special surface effects on the aluminum as shown in U.S. Pat. No. 2,653,861 may be used.

DETAILED DESCRIPTION OF THE INVENTION

My new etching treatment is generally employed on the aluminum alloys used for architectural purposes. These alloys are generally 6061, 6062, 6063, 6463, and KB45 for extrusion purposes, and 3003, 5005, 5052, 6061 for sheet etching. The process is equally satisfactory on other aluminum alloys not generally used for architectural purposes such as high strength corrosion resistant alloys used in automobile and aircraft industry. Examples are 2024 and 7075.

The aluminum metal should be carefully prepared for etching by first removing any cutting, rolling or stamping oils and greases. This is usually done by cleaning with an inhibited mild alkaline cleaner. The aluminum metal is then rinsed with water and then deoxidized by immersion or spray application in an inhibited acidic deoxidizer. Following another water rinse, the aluminum metal is ready for etching.

The alkali metal hydroxides are used in aqueous solution in my multiple stage etching process. Because of its low cost and availability sodium hydroxide is used almost exclusively for aluminum etching.

The concentration of the alkali in the first or oxidizing etching solution is within the range of about 15 to about 50 grams per liter expressed as sodium hydroxide. Higher concentrations of alkali causes rapid depletion of the oxidizing agent. A concentration of 30 to 45 grams/liter is preferred for optimum efficiency.

In the second or conventional etching solution, the caustic concentration is within the range of about 15 to about 200 grams per liter expressed as sodium hydroxide.

The etching processes of my invention are usually performed by immersion of the work pieces in the etching solution contained in a steel tank. The tanks are equipped with coils for heating and/or cooling the etching bath. Agitation of the bath is desirable to maintain a uniform concentration of the etching solution, particularly at the interface with the work piece. Good ventilation is required to dispel the caustic fumes and by-product hydrogen gas.

My process for etching aluminum using successive or multiple stage etching treatments is applicable to immersion or spray operation. Immersion is most generally used.

Generally, the first etching treatment containing the oxidizer is continued until a frosty, white appearing surface is obtained and there is a leveling of surface irregularities. The second etching treatment is continued until the aluminum surface has a uniform appearance and which has a low-reflectance. At this time nearly all surface irregularities will have been removed and the aluminum surface is frosty and white. The time for each etching treatment will be about 1 to 4 minutes in duration. However, etching treatments lasting from 1 up to 6 minutes have been used. When two successive etching treatments of 2 minutes each are employed in my process this 4 minutes of etching time compares with an etching time of 10 to 15 minutes using the convention etching processes and solutions on aluminum metal which has previously been mechanically buffed.

In commerical practice of my successive treatment etching process, after a satisfactory surface condition is obtained, it is desirable to reduce the etching time by small increments in the first etching bath and observe the results of each reduction of time on the appearance of the metal surface. The etching time is reduced in the first etching bath in this manner until a surface appearance is obtained following the second etching which is barely acceptable. This would then be the minimum time for immersion in the first etching solution. A minimum immersion time is desirable in the first etching solution which contains the oxidizer since this is the most expensive etching solution to replenish.

The sources of oxidizing agents used in the first or oxidizing etching solution for etching unbuffed aluminum are as follows:

The oxidizing agent in the alkali metal hydroxide solution is used at a concentration of about 2 to 120 grams/liter. About 2 grams/liter is the minimum concentration of oxidizing agent which will provide acceptable surface appearance. This is observed by visual inspection and/or by measuring both total and diffuse reflectivity with a Photo-Volt Reflectometer. A preferred concentration range for the oxidizing agent is about 30 to 60 grams per liter. At this concentration level the reflectance of the etched aluminum is generally non-image forming. At concentrations higher than 120 grams/liter no increased benefits are obtained.

When nitrites are employed in the first etching solution it is desirable but not necessary to have nitrate present at about 5 percent by weight of the nitrite content. The nitrate is supplied as one of the compounds selected from the group consisting of sodium nitrate, potassium nitrate and ammonium nitrate. A preferred concentration of alkali metal nitrites is 5 to 25 grams per liter of water.

The oxidizing agent may be supplied to the bath in the form of a powder or as an aqueous solution.

In operation of the etching baths it is desirable to analyze the first or oxidizing etching solution every two hours for concentration of the oxidizing agent and concentration of the caustic alkali. Replenishments are made as indicated by the analyses. The chelating agent is conveniently formulated and supplied in combination with the caustic alkali and no separate analysis for this agent is necessary.

The second or conventional etching solution should be analyzed for caustic etchant at least every four hours and replenishment of etchant should be made as indicated by the analyses. In this etching solution, the chelating agent is conveniently supplied in combination with the caustic.

The chelating agents which are efficient and compatible in the first etching solution containing the oxidizer are sorbitol, gluconic acid and glucoheptonic acid and their alkali metal salts. These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to 15 grams/liter. As a general rule, the minimum amount of chelating agent may be calculated as 2 percent by weight of the alkali metal hydroxide content.

In the conventional or non-oxidizing etching solution the chelating agent may be any agent useful for suppressing alumina precipitation during etching. Among such agents are sorbitol, gluconic acid, glucoheptonic acid, mannitol, ascorbic acid, sorbose, tannic acid, ethylenediamine tetraacetic acid, sodium chrom glucosate, diglycolic acid, picolinic acid, aspartic acid, dithiooxamide, d-glyconolactone, and l-rhamnose. These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to 15 grams/liter. As a general rule, the minimum amount of chelating agent may be calculated as 2 percent of the alkali metal hydroxide content.

The best mode of practicing my invention will be apparent from a consideration of the following examples: In these experiments various etching sequences were employed varying the composition of the etchant solution and the duration of the treatment. The etching results were measured with a PhotoVolt Reflectometer, Model 610. Total reflectivity was measured as a percent of the reflectivity obtained by setting the reflectometer at 100 on the scale using cleaned, deoxidized but unetched aluminum metal. A total reflectivity reading of 70 would represent a 30 percent decrease in total reflectivity.

Diffuse reflectivity was measured by setting the diffusion head of the PhotoVolt Reflectometer at 25 on the scale when reading the reflection from cleaned, deoxidized but unetched aluminum metal. At this reading the surface of the aluminum appears slightly frosty, slightly white with poor hide of surface imperfections. Any reading about 25 represented an increase in diffuse reflectivity. This increase would correspond to an increase in the number of corrosion pits in the aluminum metal caused by the etching process. A diffuse reflectivity of 30 represents a minimum level of diffuse reflectivity which corresponds to an acceptable level for use of the aluminum for architectural purposes. At a diffuse reflectivity of 30 there is good hide of surface imperfections and the aluminum appears uniform, frosty and moderately white in color. As used throughout this specification and claims the expression "low reflectance" means that the metal surface has a diffuse reflectivity of 30 or higher as measured in the manner set forth above. Any diffuse reflectivity reading higher than 30 also represents a satisfactory etch. At a diffuse reflectivity of about 45 virtually all surface imperfections have been hidden and the aluminum surface appears frosty and uniformly white.

In all of the experiments the aluminum metal was prepared for etching by cleaning in an inhibited mild alkaline cleaner, rinsed with with water, deoxidized in an inhibited acid deoxidizer, again rinsed with water and then etched. The same aluminum alloy and the same chelating agent were used to assure comparative results. After etching, the metal was rinsed with water, desmutted in an inhibited acid desmutter, rinsed again with water and dried in air. The etched surface was then immediately examined visually and with the reflectometer.

EXPERIMENT 1

This experiment represents a conventional etching treatment with a conventional etching solution.

Aluminum extrusions were etched for 8 minutes at 160°F in an aqueous solution containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol. The total reflectivity measured 69.9 and the diffuse reflectivity was 25. The surface appeared slightly frosty and slightly white. There was poor hide of metal imperfections. The surface was unacceptable for use in architectural purposes by present day standards.

EXPERIMENT 2

This experiment discloses the effect of multiple stage or successive short time etching treatments using conventional alkali metal hydroxide etching solutions in both stages.

Aluminum extrusions were first etched in an aqueous solution containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160°F. The extrusions were rinsed with water, desmutted and then contacted with a second etching solution by immersion containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160°F.

The total reflectivity measured 60.1 and the diffuse reflectivity was 27.5. The metal appeared slightly frosty, whiter than in Experiment 1 with fair hide of surface imperfections. This etching treatment was not satisfactory for producing aluminum for architectural purposes.

EXPERIMENT 3

This experiment represents a conventional etching treatment in which the etching solution was modified by inclusion of sodium chromate and a surfactant which provides more efficient alkali consumption and extends the bath life.

Aluminum extrusions were immersed in an aqueous solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant as disclosed in U.S. Pat. No. 3,557,000 and 0.0525 grams/liter of sodium chromate for 8 minutes at a temperature of 160°F.

The surface measured a total reflectivity of 25.5 and a diffuse reflectivity of 30.0. The surface appeared frosty, moderately white with fair hide of surface imperfections. The etched surface was considered borderline for architectural uses.

EXPERIMENT 4

This experiment shows the results of two 4 minute etches with an in-between water rinsing, desmutting and another water rinse as compared to a single 8 minute etch in the solutions used in Experiment 3. The temperature was 160°F for each etching.

After the two stage etching treatment the surface of the aluminum measured 20.0 total reflectivity and 32.5 diffuse reflectivity. The surface appeared frosty, slightly whiter than in Experiment 3 with good hide of surface irregularities. This surface was barely satisfactory for architectural uses.

EXPERIMENT 5

In this experiment the aluminum metal was subjected to successive etching treatments in which the first etching solution contained one of the oxidizing agents of this invention.

Aluminum extrusions were immersed in an aqueous solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 45 grams/liter of sodium nitrite and 0.9 grams/liter of sodium nitrate at a temperature of 160°F for four minutes. The extrusion was rinsed with water, desmutted, rinsed again with water and then etched in an etching solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at a temperature of 160°F.

After rinsing and drying the total reflectivity measured less than 1.0 and the diffuse reflectivity measured 45.0. The surface appeared frosty, white with very good hide of surface imperfections. No image was observable in the reflection. The etched surface was excellent for architectural uses.

EXPERIMENT 6

In this test sodium gluconate was used as the chelating agent. Aluminum extrusions were first immersed in an etching solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sodium gluconate, 45 grams/liter of sodium nitrite and 5.0 grams/liter of sodium nitrate for 4 minutes at 160°F.

The aluminum metal was then etched in a second solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at 160°F.

The total reflectivity was 9.0 and the diffuse reflectivity measured 38. The metal surface appeared frosty and white and had very good hide.

EXPERIMENT 7

In this test, sodium glucoheptonate was substituted for the sodium gluconate of Experiment 6 with the following results:

The total reflectivity measured 4 and the diffuse reflectivity measured 44. The metal surface appeared frosty and very white with excellent hide of surface irregularities.

EXPERIMENT 8

This experiment shows the minimum concentration level of oxidizing agent to obtain a diffuse reflection of 30 which is considered the minimum level of acceptability for architectural purposes.

Each oxidizing agent was tested in a sodium hydroxide solution at 29.4 grams/liter and 0.6 grams/liter of sorbitol at a temperature of 160°F for 4 minutes. The second etching solution contained 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate. Immersion of the work piece was 4 minutes at 160°F.

The concentration of the oxidizing agent in the first etching solution was varied until a diffuse reflectivity reading of 30 was obtained after the two etch treatment. The results appear in Table 1.

In this series of tests the upper concentration range of the oxidizing agents in the first etching bath was explored. In each test the aluminum work piece was subjected to a second etching for 4 minutes at 160°F in the second etching bath described in Experiment 8. Table II shows the composition of the first or oxidizing etching solution and the observations made on the metal surface after the second etching.

This experiment indicates the operating range of alkali metal hydroxide concentration in the first etching bath which contains the oxidizing agent.

The concentration of the caustic soda was varied in the first etching bath while the concentration of the oxidizer was identical (20 grams/liter) at the start up of each test. The consumption of the oxidizer was measured after 4 minutes of etching, at 160°F. At concentrations of 60.0 grams/liter and higher of sodium hydroxide the consumption of oxidizing agent was excessive for economic operation. The results appear in Table III.

These tests show operable temperatures in the first etching bath containing the oxidizer. The aluminum was etched for 4 minutes in the first bath at varying concentrations and temperatures followed by a second etching, after water rinsing, for 4 minutes at 160°F in a bath containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate. The results are shown in Table IV.

This experiment shows varying levels of concentration for the chelating compound in the first etching solution containing the oxidizer. Apart from the chelating compound, the first etching solution contained 45.0 grams per liter of sodium chlorite and 29.4 grams per liter of sodium hydroxide. The first etching by immersion was for 4 minutes at 160°F. After a water rinse, desmutting and second water rinse, the work pieces were etched for 4 minutes at 160°F in the second etching solution described in Experiment 11. The results appear in Table V.